The originally scheduled match between these two side was postponed in light of the Paris bombings but Oyonnax flew out of the blocks at Stade Charles-Mathon as Jeremie Maurouard and Uwa Tawalo crossed the whitewash while former Cardiff Blues star Nicky Robinson slotted three penalties to establish the hefty half-time lead.
But the introduction of Ruan Pienaar, Paddy Jackson and Nick Williams turned the game on its head as tries from Rory Scholes, the fit-again Craig Gilroy and a maiden score for young prop Kyle McCall narrowed the deficit to 23-21.
And Jackson held his nerve with a long-range 77th-minute penalty to hand Ulster the unlikeliest of wins, after which captain Rory Best admitted some harsh truths were needed in the half-time dressing room.
"It wasn't a place we wanted to be," said the hooker. "We talked fairly hard and had some fairly harsh words at half-time.
"Something we have in this group is brutal honesty. As a front-five, we had to apologise to the boys (at half-time), because we were getting done in the scrum and we had let them maul over a try.
"We were getting done in the areas in the game that require something between the ears and a bit of something in the chest. But to be fair, the boys stood up in the second half."
There was less joy for the other two GUINNESS PRO12 sides in European action over the weekend however as both Glasgow Warriors and Munster suffered defeats in France.
Reigning GUINNESS PRO12 champions Glasgow now face an uphill battle to reach the knockout stages after being comprehensively beaten 34-10 by Pool Three leaders Racing 92.
Leone Nakarawa crossed for a consolation try for the Warriors but the damage had long since been done as Ben Tameifuna, Dimitri Szarzewski, Eddy Ben Arous and Manuel Carizza secured the hosts a four-try bonus point.
And Munster are out of Europe with two Pool Four games remaining after Paul Williams, Sekou Macalou and Hugo Bonneval all dotted down for Stade Francais in a 27-7 triumph.
Stade winger Josaia Raisuqe was sent off on the stroke of half-time for gouging CJ Stander but that made little difference as, despite a late Conor Murray try preventing Munster being shut out for the first time in 21 years of European rugby, Stander concedes it was a dark day.
"It's disappointing, that's the word," said the No.8. "I know what the boys can do, we train very hard and we get to games and don't put it out on the field.
"I know what it means to the boys, what it means to me. Now it's dead rubber games from here on. The boys need to step up next week and show what the jersey means."
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